1. Field of the Invention
This invention relates to a wireless tag reader/writer control system and a wireless tag reader/writer control method for controlling the electromagnetic wave radiation time intervals of a wireless tag reader/writer for transmitting a signal to and receiving a signal from a wireless tag.
2. Description of the Related Art
When two or more than two wireless tag reader/writers radiate an electromagnetic wave simultaneously, an interference of electromagnetic waves takes place among the electromagnetic waves radiated from the wireless tag readers/writers. This problem can be avoided by allocating different frequency bands to the wireless tag readers/writers.
However, the number of frequency bands that are available to wireless tag reader/writers adapted to communicate with wireless tags for communications is limited. In other words, the number of wireless readers/writers that can be operated at the same time within a limited geographical area is limited.
With regard to this problem, it is an authorized common practice that, when a wireless tag reader/writer receives an electromagnetic wave radiated from some other wireless tag reader/writer, the former wireless tag reader/writer does not emit any electromagnetic wave (see, for example, “Notification No. 49 of the Minister of Posts and Telecommunications According to Article 49-9 of Regulations for radio equipment based on the Radio Law” (to be referred to as Non-Patent Document 1 hereinafter) and “Notification No. 385 of the Minister of Posts and Telecommunications According to Article 49-14 of Regulations for radio equipment based on Radio Law” (to be referred to as Non-Patent Document 2 hereinafter)). Techniques for selectively controlling the antennas radiating an electromagnetic wave among a plurality of antennas of wireless tag reader/writers have been proposed (see, for example, JP-A 2002-245417 (to be referred to as Patent Document 1 hereinafter)). Techniques for providing a wireless tag with a flag in order to prevent reading errors from taking place once the wireless tag is read in by a wireless tag reader/writer in order to indicate that the wireless tag is read into the internal non-volatile memory of the wireless tag reader/writer (see, for example, JP-A 2005-102215 (to be referred to as Patent Document 2 hereinafter)).
However, the techniques described in Non-Patent Document 1, Non-Patent Document 2 and Patent Document 1 cannot dissolve the problem of interference of electromagnetic waves when a plurality of wireless tag readers/writers is operated within a limited geographical area.
The technique described in Patent Document 2 is accompanied by a problem that the number of wireless tag readers/writers that are operated at the same time can seldom be detected and hence it is not practically feasible to provide flags for so many wireless tag readers/writers.